chore: update next.js

[gabrielmfern]

Closes #3489

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Summary by cubic

Upgrade next to 16.2.6 across the repo to pick up patches and keep examples in sync, closing #3489. Also bump the Tailwind e2e Next.js app from ^15 to ^16 and update the workspace catalog and lockfile.

^{Written for commit a3752f3. Summary will update on new commits.}

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Project	Deployment	Actions	Updated (UTC)
react-email	Ready	Preview, Comment	May 12, 2026 1:49pm
react-email-demo	Ready	Preview, Comment	May 12, 2026 1:49pm

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commit: a3752f3

[cubic-dev-ai]

No issues found across 5 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: This PR performs a routine patch-level upgrade of Next.js across example and e2e test configurations, including aligning the e2e test's dependency range with the rest of the monorepo, with no changes to core logic or business-critical code.}